This application claims priority from G.B. Patent Application No. 9812459.7, filed Jun. 11, 1998 and G.B. Patent Application No. 9812460.5, filed Jun. 11, 1998, the disclosures of which are incorporated by reference herein in their entirety.
The present invention relates to a device for decomposing the oxides of nitrogen and more particularly to a device for removing the oxides of nitrogen contained in gaseous effluents, such as the exhaust gases generated by automotive combustion engines. The present invention also relates to a process for decomposing these oxides, e.g. using such a device.
The use of catalytic devices in vehicle exhaust systems to remove the oxides of nitrogen (NOx) contained in the exhaust gases is well known. These devices, which are more commonly referred to as catalytic converters, typically comprise a surrounding metal shell or casing which houses a ceramic or metal monolith. The ceramic or metal monolith comprises a plurality of parallel flow channels which contain a catalytic material for catalysing the reduction of the nitrogen oxides to molecular nitrogen. In use, the exhaust gas generated by the engine is conveyed through the flow channels in the monolith so that it contacts the catalytic material.
We have now developed a device which is able to decompose the oxides of nitrogen (NOx) contained in effluent compositions, such as the exhaust gases generated by automotive combustion engines. In one embodiment, the device is able to reduce nitrogen oxides of nitrogen oxidation states greater than 2 and especially nitrogen dioxide (NO2), to molecular nitrogen. We have also developed a process for reducing nitrogen oxides of nitrogen oxidation states greater than 2 to molecular nitrogen which uses such a device.
According to a first aspect of the present invention there is provided a device for decomposing a nitrogen oxide(s) of nitrogen oxidation state greater. than 2, especially nitrogen dioxide, comprising a reaction chamber, an oxidisable material contained in the reaction chamber which in use will undergo an oxidation/reduction reaction with the nitrogen oxide(s) to generate nitrogen and an oxide(s) of the oxidisable material.
According to a second aspect of the present invention there is provided a process for decomposing a nitrogen oxide(s) of nitrogen oxidation state greater than 2, especially nitrogen dioxide, which comprises passing the nitrogen oxide(s) through a reaction chamber where it is contacted with an oxidisable material at a temperature in the range of from 100 to 1000xc2x0 C. so as to at least partially reduce the nitrogen oxide(s) to nitrogen and oxidise at least a proportion of the oxidisable material to form an oxide(s) thereof.
The present invention in its first and second aspects is particularly concerned with the treatment of gaseous effluents containing one or more nitrogen oxides of nitrogen oxidation states greater than 2, and more particularly with the treatment of gaseous effluents containing nitrogen dioxide, such as plasma effluent gas and especially the exhaust gases which are produced by automotive combustion engines such as lean burn engines. The devices used to treat such exhaust gases are more commonly known as catalytic converters.
In the device of the first aspect of the present invention the reaction chamber is preferably enclosed by a metal shell or casing. The oxidisable material contained in the reaction chamber will preferably form a layer on a suitable substrate. Conveniently, the oxidisable material will form an outer layer on a plurality of wires, particularly metal containing,wires, which are loaded into the reaction chamber. Preferably, the wires will be knitted or otherwise joined together to form an integral wire mass, although we do not exclude the possibility that the wires may be discrete or perhaps loosely consolidated to form a wool.
The reaction chamber is provided with an inlet and outlet for the effluent composition to which conduits for conveying the composition to and from the reaction chamber may be attached.
The oxidisable material which is contained in the reaction chamber is able to reduce nitrogen oxides of nitrogen oxidation states greater than 2 and especially NO2 to molecular nitrogen and is itself oxidised to form an oxide(s). As a result, the oxidisable material is gradually consumed.
Combinations of two or more oxidisable materials may be used if desired.
In a preferred embodiment, the oxidisable material contained in the reaction chamber is preferably one which will undergo an oxidation/reduction reaction with a nitrogen oxide(s) of nitrogen oxidation state greater than 2 to generate nitrogen and an oxide(s) of the oxidisable material which can be reduced in order to regenerate the oxidisable material.
In this embodiment, the device of the present invention preferably comprises means for reducing the oxide(s) so as to regenerate the oxidisable material. More particularly, the device is equipped with means for reducing the oxide(s) which is operable concurrently with the oxidation/reduction process between the oxidisable material and the nitrogen oxide(s) of nitrogen oxidation state greater than 2. In this way, the oxidisable material can be regenerated at the same time as it is consumed.
Similarly, in the process of the present invention, the oxidisable material is preferably regenerated by reducing the oxide(s) which is formed. This regeneration of the oxidisable material preferably takes place concurrently with the oxidation/reduction process.
In a particularly preferred embodiment, the oxide(s) which is formed from the oxidisable material is one which can be reduced to regenerate the oxidisable material by being subjected to an electromotive force.
Accordingly, in a third aspect of the present invention there is provided a device for decomposing a nitrogen oxide(s) of nitrogen oxidation state greater than 2, especially nitrogen dioxide, comprising a reaction chamber, an oxidisable material contained in the reaction chamber which in use will undergo an oxidation/reduction reaction with the nitrogen oxide(s) to generate nitrogen and an oxide(s) of the oxidisable material which decomposes on being subjected to an electromotive force to regenerate the oxidisable material and means for applying an electromotive force which is in electrical communication with the oxidisable material for regenerating the oxidisable material by reducing the oxide(s) which is formed.
In a fourth aspect of the present invention there is provided a process for decomposing a nitrogen oxide(s) of nitrogen oxidation state greater than 2, especially nitrogen dioxide, which comprises (1) passing the nitrogen oxide(s) through a reaction chamber where it is contacted with an oxidisable material at a temperature in the range of from 100 to 1000xc2x0 C. so as to at least partially reduce the nitrogen oxide(s) to nitrogen and oxidise at least a proportion of the oxidisable material to form an oxide(s) and (2) regenerating the oxidisable material by subjecting the oxide(s) which is formed to an electromotive force.
Although the above described devices and processes may be used to reduce any oxide of nitrogen in which the nitrogen has an oxidation state greater than 2, they are particularly concerned with the reduction of effluents containing nitrogen dioxide (NO2), and hereinafter these inventions will be explained with reference to nitrogen dioxide. However, any references to nitrogen dioxide or NO2 should be taken to include other oxides of nitrogen in which the nitrogen has an oxidation state greater than 2 unless the context requires otherwise.
The electromotive force which is preferably applied to regenerate the oxidisable material should be a direct current. The electromotive force may be continuous or pulsed and is preferably applied concurrently with the oxidation/reduction process between the oxidisable material and the NO2 so that the oxidisable material is regenerated at the same time as it is consumed.
The potential of the applied electromotive force should, of course, be sufficient to regenerate the oxidisable material by effecting the reduction of the oxide. This in turn will depend on the oxidation/reduction potential of the oxidisable material. However, oxidisable materials having low oxidation/reduction potentials are preferred and, in general, an applied electromotive force in the range of from 12 to 48 volts is suitable. When the present invention is employed in automotive applications, it is desirable to employ an oxidisable material having an oxidation/reduction potential below 12V since this represents the voltage capacity of a typical vehicle battery.
The oxidisable material is preferably a metal and more preferably is a metal having a low oxidation reduction potential, e.g. below 12 volts. Accordingly, in a preferred embodiment of the present invention the oxidation/reduction reaction with the NO2 results in the production of a metal oxide(s) which is then converted back to the metal by the application of an electromotive force.
Suitable metals include tin, gold, gallium, indium, copper, zinc, platinum and palladium as well as metal alloys formed from these metals, such as tin/copper alloys. Suitable tin/copper alloys are those comprising from 1 to 99 weight % of tin and from 1 to 99 weight % of copper, preferably from 30 to 99 weight % of tin and from 1 to 70 weight % of copper and more preferably from 50 to 95 weight % of tin and from 5 to 50 weight % of copper.
An especially preferred metal is tin which on oxidation can form tin (II) oxide (SnO) and tin (IV) oxide (SnO2), both of which can be readily reduced to metallic tin. Tin (II) oxide can be reduced to metallic tin with the application of a 1.24 volt electromotive force (e.m.f.) and tin (IV) oxide requires a 2.5 volt e.m.f. to be reduced to tin. When tin is employed as the oxidisable material, the applied electromotive force is typically at least 3 volts.
The tin is preferably combined with a co-metal(s) which forms a catalytic composition with the tin. Suitable metals for this purpose may be selected from rhodium, platinum and palladium, with rhodium being particularly preferred. Without wishing to be bound by any theory, it is believed that the co-metal facilitates the release of oxygen from the tin oxides which are formed, e.g. by feeding electrons into the oxide molecules, and/or is able to absorb the oxides of nitrogen. Thus, other materials which are able to perform one or both of these functions may be used in place of the co-metal. Moreover, if other oxidisable metals are used instead of tin, these may be combined with a co-metal or other material which is able to facilitate the release of oxygen from the metal oxide(s) which is formed and/or which is able to absorb the oxides of nitrogen.
When the co-metal is rhodium, the mole ratio of tin to rhodium is preferably in the range of from 2:1 to 1:4, more preferably around 1:2.
Mixtures of two or more co-metals may be used if desired.
The tin or tin/co-metal combination is also preferably formulated into a composition together with one or more electrically conducting metal oxides which have variable oxidation states. Suitable electrically conducting metal oxides of this type include zirconia and the oxides of the lanthanides, such as the oxides of lanthanum (La), ytterbium (Yb), yttrium (Y), neodymium (Nd), gadolinium (Gd), praseodymium (Pr) and cerium (Ce). Ceria and zirconia are particularly preferred oxides, and combinations of these two oxides are especially preferred.
If other metal or metal/co-metal combinations are used, these are also preferably formulated into a composition together with an electrically conducting oxide which has variable oxidation states.
The amount of oxidisable material which is used in the present invention will preferably be sufficient to reduce substantially all and more preferably all of the NO2. In this way, the effluent composition which exits the reaction chamber will be substantially free of NO2. The amount of oxidisable material which is used will depend, inter alia, on the nature of the oxidisable material, the surface area of the oxidisable material, the amount of effluent to be processed per unit time and the concentration of NO2 in the effluent. Preferably, the loading of the oxidisable material in the reaction chamber will be such as to provide from 20 to 300 mg of the material per cm3 of reaction chamber, more preferably from 70 to 250 mg/cm3 and particularly preferably from 100 to 200 mg/cm3, e.g. around 160 mg/cm3. When the present invention is being applied to the treatment of vehicle exhaust gases, the amount of the oxidisable material should be sufficient to process exhaust gas having a weight hourly space velocity of up to 100,000 per hour (hxe2x88x921). This amount can be readily determined by one skilled in the art by means of routine trials.
The application of the electromotive force is preferably achieved by making the oxidisable material, and any other material(s) with which it is combined, a component in a solid state electrolytic cell which also comprises a metallic component, a solid state electrolyte component arranged in electrical communication with both the metallic component and the oxidisable material, and means for providing an electrical connection to a voltage source.
For the avoidance of doubt, by a metallic component we mean a component which comprises a metal so that it is electrically conducting, but which is not necessarily made exclusively of metal.
In the cell, the metallic component will function as the cell""s positive electrode and will also provide a substrate for the solid state electrolyte and the oxidisable material. The oxidisable material, and any other material with which it is combined, will function as the cell""s negative electrode and will, therefore, experience a negative bias on application of the electric current.
The metallic component of the electrolytic cell is preferably provided by a plurality of metal containing wires which may be discrete or perhaps loosely consolidated to form a wool. However, in a preferred embodiment the wires will be knitted or otherwise joined together to form an integral wire mass. Other suitable constructions for the metallic component will be apparent to those skilled in the art.
In a preferred embodiment, the metallic component of the electrolytic cell comprises an inner layer or core of a metal and an outer layer or coating of an inorganic oxide which will protect the metal inner layer from the oxidising environment and also provide a suitable surface for the subsequently applied electrolyte. When the metallic component comprises a plurality of wires, as is preferred, each wire will preferably comprise a core of metal and an outer layer of an inorganic oxide.
In an especially preferred embodiment, the metallic component comprises an inner layer or core which is substantially composed of an iron/chromium (Fe/Cr) alloy and an outer layer, on which the electrolyte is deposited, which is substantially composed of alumina and particularly xcex1-alumina. The substrates of this especially preferred embodiment can be prepared by heating an alloy of Fe, Cr and aluminium. (Al), such as Fecralloy, to a temperature of between 600 and 1000xc2x0 C. in air for about 1 to 4 hours, e.g. around 1000xc2x0 C. for about 2 hours. In this process, the aluminium tends to migrate to the surface where it is oxidised to form alumina.
The solid state electrolyte will form a layer on the metallic component and will typically comprise a mixture of one or more semi-conducting oxides. In a preferred embodiment, the electrolyte comprises alumina or zirconia, especially zirconia, and one or more oxides of the lanthanides (i.e. rare earth oxides) as a dopant, such as an oxide of lanthanum (La), ytterbium (Yb), yttrium (Y), neodymium (Nd), gadolinium (Gd), praseodymium (Pr) and cerium (Ce). Electrolytes comprising a mixture of zirconia and ceria are particularly preferred with electrolytes comprising a mixture of zirconia, ceria and gadolinia being especially preferred.
The electrolyte will typically comprise from 50 to 80 mole %, particularly from 60 to 75 mole % of the zirconia or alumina and from 20 to 50 mole %, particularly from 25 to 40 mole % of the one or more lanthanide oxides.
The application of an electrolyte layer to the metallic component is preferably achieved by coating a precursor layer from a sol and then heat treating the precursor layer to form the electrolyte layer. The sol is preferably made up by dissolving or dispersing salts or oxides of the various metals contained in the electrolyte layer in the desired proportions in a concentrated nitric acid (c.HNO3)/water mixture. The c.HNO3/water mixture preferably comprises a 1:1 mixture by volume of the c.HNO3 and water. Suitable salts include the carbonates, oxalates, halides, hydroxides and nitrates, with carbonates and nitrates being preferred. Heating may be necessary to effect thorough digestion of the various metal compounds in the c.HNO3/water mixture. When a sol having the desired homogeneity and consistency is obtained, it can be coated onto the metallic component using any appropriate coating technique, such as dipping, spraying or brushing, dipping being a preferred technique. Once the coating has been applied to the metallic component, it is dried and then baked at a temperature in the range of from 300 to 800xc2x0 C., preferably in the range of from 300 to 600xc2x0 C. and particularly, in the range of from 300 to 500xc2x0 C., e.g. around 400xc2x0 C., to form the electrolyte layer.
In a particularly preferred embodiment, zirconium carbonate is first dissolved in a c.HNO3/water mixture, preferably a 1:1 v/v mixture, followed by the nitrate salt(s) of the selected rare earth metal(s). The resulting composition is then heated to a temperature in the range of from 60 to 100xc2x0 C. to digest the compounds in the c. HNO3/water mixture. Preferably, temperatures in the range of from 70 to 90xc2x0 C., e.g. around 80xc2x0 C., are used to digest the various compounds in the c. HNO3/water mixture. The heating is typically continued for about an hour. After cooling, the sol is coated onto the substrate, e.g. by dipping, the coating dried and then baked at around 400xc2x0 C. to form the electrolyte layer.
The amounts of the various components making up the sol can be selected to result in the formation of an electrolyte layer having the desired ratio of components.
Without wishing to be bound by any theory, it is believed that the solid state electrolyte removes O2xe2x88x92 ions that have reacted with the oxidisable material as a result of the reaction between that material and the NO2. Therefore, the thickness of the electrolyte layer will depend, inter alia, on the number of moles of NO2 to be processed per unit time. This thickness can be readily determined by one skilled in the art.
The oxidisable material, which must be in electrical communication with the solid state electrolyte, will preferably form a continuous layer over the solid state electrolyte layer. This layer should be porous in order to allow for the migration of O2xe2x88x92 ions from the oxidisable material to the electrolyte.
The layer of oxidisable material may be applied using coating techniques known in the art. For example, when the oxidisable material is a metal, a preferred technique for applying the layer of oxidisable material is electrodeposition. In this technique, a suitable solution containing the metal as a salt, e.g. an aqueous solution of the nitrate or chloride salt, is deposited as a layer on the electrolyte under high voltage, e.g. around 12 volts. The applied voltage used in electrodeposition will convert the metal salt to the metal and will also ensure that the layer of oxidisable material which is deposited is porous.
When the oxidisable metal is combined with a co-metal and an electrically conducting metal oxide having variable oxidation states, an electrode layer comprising these three components is conveniently formed by applying a precursor layer from a sol and then heat treating this layer to form the electrode layer. The sol is preferably made up by dissolving or dispersing salts or oxides of the various metals contained in the electrode layer in the desired proportions in a concentrated nitric acid (c.HNO3)/water mixture. The c.HNO3/water mixture preferably comprises a 1:1 mixture by volume of the c.HNO3 and water. Suitable salts include the carbonates, oxalates, halides, hydroxides and nitrates. Heating may be necessary to effect thorough digestion of the various metal compounds in the c.HNO3/water mixture. When a sol having the desired homogeneity and consistency is obtained, it can be coated onto the electrolyte layer using any appropriate coating technique, such as dipping, spraying or brushing, dipping being a preferred technique. Once the coating has been applied to the electrolyte layer, it is dried and then baked at a temperature in the range of from 300 to 800xc2x0 C., preferably in the range of from 300 to 600xc2x0 C. and particularly in the range of from 300 to 500xc2x0 C., e.g. around 400xc2x0 C., to form the electrode layer.
In a particularly preferred embodiment, zirconium carbonate is first dissolved in a c.HNO3/water mixture, preferably a 1:1 v/v mixture, followed by cerium nitrate if the final electrode layer is to contain ceria. The resulting composition is then heated to a temperature in the range of from 60 to 100xc2x0 C. to digest the compounds in the c.HNO3/water mixture. Preferably, temperatures in the range of from 70 to 90xc2x0 C., e.g. around 80xc2x0 C., are used to digest the various compounds. The heating is typically continued for about an hour. Halide, particularly chloride salts of the co-metal and oxidisable metal are then added in turn to the sol with stirring, and once the sol has cooled it is coated onto the electrolyte layer, e.g. by dipping, the coating dried and then baked at around 400xc2x0 C. to form the electrode layer. If the oxidisable metal and co-metal are tin and rhodium respectively, tin (II) chloride (SnCl2) and rhodium (III) chloride (RhCl3) are preferably used in the formation of the sol.
The regeneration of the oxidisable material in the electrolytic cell is achieved by applying a direct current thereto which is equal to or greater than the reduction potential of the oxide, e.g. metal oxide, which is formed. The positive terminal of the voltage source is connected to the metallic component and the negative terminal is connected to the electrode component comprising the oxidisable material.
Without wishing to be bound by any theory, it is believed that the reduction of the oxide back to the oxidisable form liberates O2xe2x88x92 ions which are removed by the solid state electrolyte as a result of the concentration gradient generated by the applied electromotive force. The electromotive force flowing through the metallic component also provides for conversion of the O2xe2x88x92, ions to molecular oxygen which is released.
When the device of the first aspect of the present invention is used in automotive applications, the voltage source is conveniently the vehicle battery.
In a preferred embodiment, the electrolytic cell also comprises a porous component interposed between the metallic component and the solid state electrolyte which facilitates the release of the oxygen gas which is generated. This component preferably comprises a layer on the metallic component onto which the electrolyte layer is deposited. In order to ensure electrical communication between the metallic component and the solid state electrolyte, the additional porous component should be electrically conducting, and for this reason is generally fabricated from a metal.
The additional porous component is preferably made of gold and is conveniently formed by electrodeposition in which the gold is deposited as a layer on the metallic component from a solution containing the gold as a salt, e.g. an aqueous solution of the nitrate or chloride salt, under high voltage, e.g. around 12 volts. The applied voltage used will convert the metal salt to the metal and will also ensure that the gold layer has the desired porosity. Alternatively, the porous gold layer may be formed by coating the metallic component with a gold/aluminium alloy to form a layer which is then treated with a chemical reagent such as sodium hydroxide to etch out the aluminium and leave a porous gold layer behind.
In the process of the second aspect of the present invention, the reaction chamber containing the oxidisable material is maintained at a temperature in the range of from 100 to 1000xc2x0 C. Preferred operating temperatures will depend, inter alia, on the nature of the oxidisable material, on the operating temperature of the electrolyte and on the use to which the process is being put. However, when the oxidisable material is a metal preferred operating temperatures are in the range of from 200 to 600xc2x0 C., more preferably in the range of from 200 to 500xc2x0 C. and particularly in the range of from 300 to 400xc2x0 C., e.g. around 350xc2x0 C. When the process is employed in automotive applications, the required operating. temperatures are generally maintained by the hot exhaust gas entering the reaction chamber so that a discrete heating source is unnecessary.
The residence time for the effluent composition in the reaction chamber containing the oxidisable material is typically in the range of from 1 to 50 milliseconds, preferably in the range of from 1 to 20 milliseconds, more preferably in the range of from 2 to 10 milliseconds and particularly in the range of from 2 to 8 milliseconds, e.g. about 4 milliseconds.
In a particularly preferred embodiment, the reaction chamber containing the oxidisable material is associated with a further reaction chamber which is arranged upstream of the reaction chamber containing the oxidisable material. This further reaction chamber contains an oxidation catalyst which is able to oxidise nitric oxide (NO) to nitrogen dioxide (NO2) or other nitrogen oxides having a nitrogen oxidation state greater than 2. Thus, effluent compositions such as exhaust gases which contain both NO and NO2 can be passed through the reaction chamber containing the oxidation catalyst where the NO is oxidised to NO2 and then through the reaction chamber containing the oxidisable material where NO2 is reduced to nitrogen.
According to a fifth aspect of the present invention there is provided a device for decomposing nitric oxide (NO) or a nitric oxide (NO)/nitrogen dioxide (NO2) mixture contained in an effluent composition comprising in series a first reaction chamber which contains an oxidation catalyst which is able to oxidise nitric oxide to yield nitrogen dioxide and a second reaction chamber which contains an oxidisable material which is able to reduce nitrogen dioxide to yield nitrogen.
The device of the fifth aspect of the present invention comprises first and second reaction chambers which are arranged in series. When the device is in use, the effluent composition is initially conveyed to the first reaction chamber where the oxidation catalyst contained in the chamber oxidises NO contained in the effluent composition to yield NO2. The oxidised effluent composition then exits the first reaction chamber and passes into the second reaction chamber where the oxidisable material reduces the NO2 contained in the effluent composition to yield molecular nitrogen which can be safely discharged into the atmosphere.
In the device, the first and second reaction chambers are preferably enclosed by a metal shell or casing. The oxidation catalyst contained in the first reaction chamber will preferably form a layer on a metal or ceramic substrate. Conveniently, the oxidation catalyst will form an outer layer on a plurality of wires, particularly metal containing wires, which are loaded into the reaction chamber. Preferably, the wires will be knitted or otherwise joined together to form an integral wire mass, although we do not exclude the possibility that the wires may be discrete or perhaps loosely consolidated to form a wool.
Both reaction chambers are provided with an inlet and outlet for the effluent composition to which conduits for conveying the composition to and from the reaction chambers may be attached. The first and second reaction chambers may be enclosed by the same metal casing and be separated by a partitioning wall which effectively divides the single large chamber enclosed by the metal casing into two. In this embodiment, the partition will be provided with an aperture for allowing the effluent composition to pass from the first to the second reaction chamber.
According to a sixth aspect of the present invention there is provided a process for decomposing nitric oxide (NO) or a nitric oxide (NO)/nitrogen dioxide (NO2) mixture contained in an effluent composition which comprises passing the effluent composition through a first reaction chamber where it is contacted with oxygen in the presence of an oxidation catalyst at a temperature in the range of from 200 to 800xc2x0 C. so as to oxidise at least a proportion of the nitric oxide contained in the effluent composition to nitrogen dioxide and then through a second reaction chamber where it is contacted with an oxidisable material at a temperature in the range of from 100 to 1000xc2x0 C. so as to at least partially reduce the nitrogen dioxide to yield nitrogen.
Typically, the effluent composition which enters the first reaction chamber will contain both NO and NO2 so that the effluent composition which enters the second reaction chamber will contain the NO2 originally contained in the effluent composition as well as that generated in the first reaction chamber.
For the avoidance of doubt, although the device and process of the fifth and sixth aspects of the present invention have been defined in relation to NO and NO2, they may be applied to the treatment of nitric oxide (NO) and any oxide of nitrogen in which the nitrogen has an oxidation state greater than 2. For example, the original effluent composition may contain a proportion of nitrogen oxides of nitrogen oxidation states greater than 2 other than NO2. Moreover, a proportion of the NO may also be oxidised in the first reaction chamber to nitrogen oxides of nitrogen oxidation states greater than 2 other than NO2. However, typically, the oxides of nitrogen contained in the original effluent composition will substantially comprise NO and NO2 and the NO which is oxidised in the first reaction chamber will be substantially oxidised to NO2.
In the description which follows, we will continue to use the terms nitrogen dioxide and NO2. However, we do not intend to exclude other oxides of nitrogen in which the nitrogen has an oxidation state greater than 2 unless the context requires otherwise.
The inventions of the fifth and sixth aspects, are particularly concerned with the treatment of gaseous effluent compositions containing NO and NO2, such as plasma effluent gas. More especially, they are concerned with the treatment of exhaust gases produced by automotive combustion engines, particularly lean burn engines, which contain NO and NO2.
The oxidation catalyst which is contained in the first reaction chamber is able to oxidise NO to NO2 and in a preferred embodiment will oxidise substantially all and more preferably all of the NO. In this way, the is effluent composition which enters the second reaction chamber contains NO2, but is substantially free of NO. A suitable oxidation catalyst may be selected from the oxidation catalysts known in the art.
A preferred oxidation catalyst is one comprising an inorganic oxide support which in use provides a source of O2xe2x88x92 ions and which is doped or impregnated with one or more metals and/or metal compounds, e.g. metal oxides, which are able to catalyse the reaction between NO and atomic oxygen to generate NO2. The support typically comprises from 85 to 98 weight % and the metal/metal compound from 2 to 15 weight % of the oxidation catalyst. Preferably, the support comprises from 92 to 98 weight %, more preferably from 94 to 98 weight % and the metal/metal compound impregnating the support from 2 to 8 weight %, more preferably from 2 to 6 weight % of the oxidation catalyst.
The inorganic oxide support preferably comprises an oxide of a lanthanide (i.e. a rare earth oxide) and more preferably comprises a mixture of a lanthanide oxide and zirconia or alumina, especially. zirconia. Suitable lanthanide oxides include the oxides of cerium (Ce), praseodymium (Pr), gadolinium (Gd), ytterbium (Yb), yttrium (Y) and neodymium (Nd), such as Yb2O3, Gd2O3, Y2O3, Ce2O3 and Nd2O3. Mixtures of lanthanide oxides may be used if desired. Preferred oxides are the oxides of cerium and praseodymium, and in a particularly preferred embodiment the support comprises a mixture of zirconia and an oxide of praseodymium. Preferred supports comprise from 50 to 90 mol % of the zirconia or alumina and from 10 to 50 mol % of the lanthanide oxide.
The support is preferably impregnated with at least one metal or metal oxide selected from palladium, nickel, platinum, rhodium, silver, ruthenium, cobalt, iron, molybdenum, tungsten and the oxides of these metals. Combinations of a metal and metal oxide may also be employed.
Preferably, the support is not only impregnated with one of the above metals or metal oxides, but is also impregnated with a metal or metal compound which is able to function as an electron donor. Suitable electron donating metals and metal compounds include the transition metals, aluminium, zinc and the oxides of these metals, particularly zinc and zinc oxide.
In a preferred embodiment, the support is impregnated with platinum, palladium, a platinum/zinc mixture, a palladium/zinc mixture or an oxide derivative of these materials. In a particularly preferred embodiment, the impregnant is a mixture comprising (A) a metal or metal oxide selected from platinum, palladium and the oxides thereof and (B) zinc or zinc oxide, with mixtures of palladium and zinc or their oxides being especially preferred. In this embodiment, the molar ratio of platinum or palladium to zinc is preferably 1:2.
Oxidation catalysts comprising a support of a rare earth oxide, e.g. praseodymia, together with zirconia or alumina and a platinum or palladium oxide impregnant are conveniently prepared by impregnating the support material with a platinum or palladium salt and then calcining the impregnated support in air at elevated temperatures to convert the salt into the oxide. When the impregnant is a mixture containing the oxides of platinum and zinc or palladium and zinc, the catalyst is conveniently prepared by impregnating the support with a mixture of platinum and zinc or palladium and zinc salts in the appropriate ratio and then calcining as before. The calcination temperature will depend, inter alia, on the type of salt which is used. Suitable salts include the carbonates, oxalates and halides, with nitrates being preferred. When nitrates are used, the calcination temperature is typically in the range of from 500 to 800xc2x0 C., more preferably in the range of from 550 to 700xc2x0 C. and particularly around 600xc2x0 C.
As stated above, the oxidation catalyst preferably comprises a layer, particularly a porous layer, on a metal or ceramic substrate. Substrates comprising an inner layer or core of a metal and an outer layer or coating of an inorganic oxide are particularly preferred, since the inorganic oxide tends to protect the metal inner layer and also provides a suitable surface for the subsequently applied oxidation catalyst. In an especially preferred embodiment, the substrate, which preferably comprises a plurality of wires, comprises an inner layer or core which is substantially composed of an iron/chromium (Fe/Cr) alloy and an outer layer, on which the oxidation catalyst is coated, which is substantially composed of alumina and particularly xcex1-alumina. The substrates of this especially preferred embodiment can be prepared as described previously.
When the oxidation catalyst is a porous layer coated on a substrate, e.g. a wire substrate, a preferred technique for applying the catalyst layer to the substrate is a coating technique in which a precursor layer is applied using a sol and then heat treated. The sol is preferably made up by dissolving or dispersing salts or oxides of the various metals contained in the oxidation catalyst in the desired proportions in a concentrated nitric acid (c.HNO3)/water mixture. The c.HNO3/water mixture preferably comprises a 1:1 mixture by volume of the c.HNO3 and water. Suitable salts include the carbonates, oxalates, halides, hydroxides and nitrates, with nitrates being preferred. Heating may be necessary to effect thorough digestion of the various metal compounds in the c.HNO3/water mixture. When a sol having the desired homogeneity and consistency is obtained, it can be coated onto the substrate using any appropriate coating technique, such as dipping, spraying or brushing, dipping being the preferred technique. Once the coating has been applied to the substrate, it is dried and then baked at a temperature in the range of from 300 to 800xc2x0 C., preferably in the range of from 300 to 600xc2x0 C. and particularly in the range of from 300 to 500xc2x0 C., e.g. around 400xc2x0 C., to form the catalyst layer.
In a particularly preferred embodiment, zirconium carbonate is first dissolved in a c.HNO3/water mixture, preferably a 1:1 v/v mixture, followed by cerium or praseodymium nitrate. Palladium or platinum nitrate is then added to the mixture optionally together with zinc nitrate and the resulting composition is then heated to a temperature in the range of from 60 to 100xc2x0 C. to digest the compounds in the c.HNO3/water mixture. Preferably, temperatures in the range of from 70 to 90xc2x0 C., e.g. around 80xc2x0 C., are used to digest the various compounds in the c. HNO3/water mixture. The heating is typically continued for about an hour. After cooling, the sol is coated onto the substrate, e.g. by dipping, the coating dried and then baked at around 400xc2x0 C. to form the catalyst layer.
The amounts of the various components making up the sol can be selected to result in the formation of an oxidation catalyst having the desired ratio of components.
The NO and O2 molecules tend to compete for absorption sites on the surface of the oxidation catalyst. In consequence, when the partial pressure of oxygen in the first reaction chamber is greater than that of NO, as is the case with exhaust gases generated by automotive combustion engines, a proportion of the NO entering the first reaction chamber may not be oxidised in that chamber. However, if the oxidation catalyst comprises a layer on an electrically conducting substrate, such as a metal substrate or a substrate comprising an inner layer or core of a metal and an outer layer or coating of an inorganic oxide, an electromotive force with a negative bias can be passed through the substrate and thus through the oxidation catalyst in order to promote the absorption of NO and discourage the absorption of O2 on the catalyst surface. This process of preferentially encouraging the absorption of one or more components of a reactive gas stream onto the surface of a catalyst while simultaneously discouraging the absorption of one or more other components is of more general applicability.
Accordingly, in a further aspect of the present invention there is provided a device for effecting a chemical reaction comprising a reaction chamber, a catalyst material for the chemical reaction which is contained in the reaction chamber and which forms a layer or coating on an electrically conducting, e.g. a metal or metal containing substrate and means for applying an electromotive force to the catalyst material via the electrically conducting substrate so as to modify the absorption characteristics of the catalyst material.
The present invention also provides a process for modifying the absorption characteristics of a catalyst material contained in a reaction chamber through which a reactant stream containing one or more reactants wand optionally one or more further components is being passed, which process comprises applying an electromotive force to the catalyst material so as to promote the absorption of one or more of the reactants and discourage the absorption of one or more of the other reactants or other components.
The oxidation catalyst or other catalytic material is conveniently coated on an electrically conducting substrate, such as an electrically conducting plate or an arrangement of electrically conducting wires,. which is sandwiched between two electrically conducting plates and insulated therefrom. The catalytic assembly is also provided with means for connection to a voltage source. In use, the connections to the voltage source are such as to supply the central electrically conducting substrate which supports the catalyst with a negative charge and the outer electrically conducting plates with a positive charge. The insulation between the central and outer electrically conducting components will prevent shorting. The substrate supporting the catalyst and the outer electrically conducting plates conveniently comprise a metal core and an outer layer or coating of an inorganic oxide, such as an alumina coated Fecralloy alloy. The preparation of these latter substrates has been described supra.
The amount of oxidation catalyst contained in the first reaction chamber will depend, inter alia, on the nature of the catalyst, the surface area of the catalyst, the amount of effluent composition to be processed per unit time and the concentration of NO in the effluent composition. Preferably, the loading of the oxidation catalyst in the first reaction chamber will be such as to provide from 1 to 10 mg of catalyst per cm3 of reaction chamber, more preferably from 1 to 5 mg/cm3 and particularly preferably from 1 to 3 mg/cm3, e.g. around 2 mg/cm3. When the present invention is being applied to the treatment of vehicle exhaust gases, the amount of catalyst should be sufficient to process exhaust gas having a weight hourly space velocity of up to 100,000 per hour (hxe2x88x921). This amount can be readily determined by one skilled in the art by means of routine trials.
In the process of the present invention, the first reaction chamber is maintained at a temperature in the range of from 200 to 800xc2x0 C. Preferred operating temperatures in the first reaction chamber will depend, inter alia, on the nature of the oxidation catalyst and on the use to which the process is being put. However, preferred operating temperatures are in the range of from 250 to 600xc2x0 C., more preferably in the range of from 250 to 450xc2x0 C. and particularly in the range of from 350 to 450xc2x0 C., e.g. 400xc2x0 C. When the process of the present invention is employed in automotive applications, the required operating temperatures in the first reaction chamber are generally maintained by the hot exhaust gas entering the reaction chamber so that a discrete heating source is unnecessary.
The residence time for the effluent composition in the first reaction chamber is typically in the range of from 1 to 50 milliseconds, preferably in the range of from 1 to 20 milliseconds, more preferably in the range of from 2 to 10 milliseconds and particularly in the range of from 2 to 8 milliseconds, e.g. about 4 milliseconds.